1. Field of the Invention
The present invention is directed, in general, to power conversion and, more specifically, to a power converter having a capacitively coupled feedback circuit for minimizing circuit costs and providing bi-directional communication in the power converter.
2. Description of the Related Art
A power converter is a power processing circuit that converts an input voltage or current waveform into a specified output voltage or current waveform. A switched-mode power converter is a frequently employed power converter that converts an input voltage into a specified output voltage. Power converters generally include power transfer components (e.g., transformers and switches) and control circuitry which governs the operation of the power transfer components to achieve desired operating characteristics.
It is often important (e.g., for safety reasons) that the primary and secondary sides of the converter be isolated from one another. Safety regulations (such as ULTM. requirements in the United States) often require some isolation between the power supply mains and electronic equipment. As a result, the transfer of control information from the secondary to primary sides is usually done via an isolation device (e.g., a transformer in which two windings are coupled by a permeable core, an opto-coupler, or a discrete light emitting device located adjacent to a discrete light detecting device) which is included in the control circuitry.
FIG. 1 is an illustration of a prior art power converter 100 which illustrates one technique for providing feedback from secondary to primary by using a second (small-signal) transformer T2. The voltage Vin on the primary side can be either an AC or DC voltage. The feedback control signal can be either amplitude-modulated (as in the case of the Unitrode UC3901) or pulse-triggered (as in the case of Intersil ICL7675/7676). In the latter arrangement, compensation could be implemented entirely on the secondary side, with only the final PWM signal being sent to the primary side.
FIG. 2 is an illustration of a prior art power converter 200 which utilizes an opto-coupler 202 for providing DC isolation for the control loop. Opto-couplers are typically used where safety regulations require galvanic isolation between the AC line voltage and the outputs of a switch-mode power supply. In the circuit of FIG. 2, the power transformer T1 is used to transfer the desired energy from the primary side to the secondary side, while maintaining DC isolation. The optical coupler 202 is used to feed the output signal back to the primary side. The voltage Vin on the primary side can be either an AC or DC voltage. The reference voltage Vref and a compensator 204 (or filter) is implemented on the secondary side. Additional compensation is usually necessary on the primary side. The use of opto-couplers is applicable to many power converter topologies, such as forward converter, flyback converter, etc.
One disadvantage associated with transformer isolation, as shown in FIG. 1, is that the isolation transformer, T2, is optimally designed to transfer high frequency signals (e.g., gate signals), and is ill-suited to transfer direct current (DC) signals whose characteristic frequencies can be highly variable. Another disadvantage associated with transformer isolation is that the transformer is susceptible to saturation. To prevent this occurrence, reset circuitry is often required.
Optical coupler isolation, as shown in FIG. 2 introduces its own unique set of disadvantages, such as low bandwidth (due to its limited frequency response), non-linearity and poor temperature stability and unidirectional communication capability. Accordingly, additional compensation circuits are needed on the secondary side.
A further disadvantage associated with both prior art approaches for providing feedback is that both methods require relatively expensive isolation devices (e.g., transformers, opto-couplers).
A still further disadvantage associated with the prior art approaches is that it is very difficult to design the compensator and reference voltage on the primary side. As such, the reference circuit and compensator cannot be integrated into the primary controller. This is especially true when the primary controller is implemented in a microcontroller or DSP. In this case, a filter and a reference voltage are still required on the secondary side. This limitation increases product costs and reduces flexibility.
Accordingly, there is a need in the art for a feedback approach that overcomes the afore-stated disadvantages of the prior art.
The present invention is embodied in a fly-back converter employing feedback and electrical isolation in a unique fashion using inexpensive components. While a fly-back converter is used to illustrate the teachings of the present invention, the principles of the invention may be applied to other power converter topologies. Instead of using optical or transformer coupling to provide electrical (DC) isolation as is well known in the prior art, the present invention uses a capacitive coupling network in the feedback loop.
The capacitive coupling network or circuit of the present invention has applicability to digital and analog embodiments. In an analog embodiment, the capacitive coupling network provides a more effective solution in terms of both cost and size than prior art optical and transformer coupling networks. In a digital embodiment, the capacitive coupling network provides a flexible solution than afforded by the prior art by providing a bi-directional communication capability between the primary and secondary sides of the converter for transmitting control data and commands there-between. Further, the digital embodiment advantageously removes the need for either the primary or secondary side micro-controller as used in the prior art. A still further advantage is that the digital information is transferred bi-directionally substantially without corruption therefore there is no linearity or temperature sensitivity problems. It is further noted that both the analog and digital implementations maintain DC isolation.